Elimination of white layer in nitrided steel



United States Patent G ELIMINATIQN F WHITE LAYER IN NITRHDED STEEL Filed Nov. 10, 1958, Ser. No. 772,708

6 Claims. c1. 148-16.6)

No Drawing.

This invention relates to the elimination of the white layer in nitrided steel and refers more particularly to a process for removing the so-called white layer from ferrous metal which has been surface hardened by the process of nitriding, comprising coating the metal with a material to prevent loss of nitrogen from the metal on application of heat thereto and application of heat to the metal.

The surface hardening of steel by the process of nitriding is accomplished commercially by heating steels containing special alloying elements such as aluminum, chromium and molybdenum in an atmosphere of ammonia gas. The depth of the surface hardening obtained by this process is a function of the temperature of the metal and the time it is in contact with the ammonia gas. In present practice the temperature is held between 925 and 975 for periods from ten to one hundred hours. Part of the ammonia gas in contact with the hot metal decomposes into nitrogen and hydro-gen in accordance with the formula, NH =N+3H. The hydrogen formed apparently passes immediately to the molecular state. A part of the nitrogen formed reacts to form inert N The remainder of the nitrogen is absorbed by the metal and forms nitrides with the iron and special alloying elements in the steel at the surface of the metal. The nitrides of the special alloying elements form along the crystal planes of the iron producing a hardened case on the metal.

The formation of these nitrides on the surface of ferrous metals heated in an ammonia atmosphere is not uniform with depth. Two relatively distinct layers are formed. In the outer layer substantially all of the nitride forming elements have been converted to nitrides. This layer appears white when viewed through a microscope after it has been given a nital etch. For this reason it is called the white layer in the metal hardening trade and is therefore referred to as such in the consideration of the present invention. Beneath the white layer a second layer of hardened metal exists. In this region most of the special elements in the steel, but not the iron, are nitrides.

The outer, or white layer, is hard and very brittle. It is objectionable in the use of most nitride hardened materials and therefore must be removed before the case hardened metal is serviceable. The white layer is relatively thin in comparison to the inner hardened layer. Therefore it has been past practice to remove the white layer by a grinding operation. Also, it has been suggested that the thickness of the white layer may be controlled to some degree by the regulation of the extent of the dissociation of the ammonia gas in contact with the metal on a predetermined time basis, thus reducing the problem of removing the white layer.

The grinding operation used commercially to remove the white layer from nitrided metal is costly in time and labor. Special machinery is required for this operation and the grinding of a hardened case may produce untrue or otherwise objectionable surfaces. Furthermore the 2,960,42l Patented Nov. 15, 1960 control of the thickness of the White layer of nitrided metal by regulation of the extent of dissociation of ammonia gas in contact with the metal during the nitriding process so as to minimize the grinding required to remove this layer has been found to be unsatisfactory. The results of such a procedure are not considered to be sulficiently predicable to solve the problem of commercial removal of the white layer in nitrided steel.

Therefore it is one of the essential objects of this invention to provide a process for the removal of the white layer in nitrided steel.

It is another object to provide a predicable process for the commercial removal of the white layer in nitrided steel.

Another object is to provide a process by which the brittle outer layer of nitrided steel may be removed after the nitriding has been completed.

Another object is to provide a process for removing the white nitride layer from nitrided ferrous metal comprising the steps of coating the metal with a material to prevent the loss of nitrogen from the metal on application of heat thereto, applying heat to the metal for a period depending on the thickness of the White layer and subsequently removing the coating.

Still another object is to provide a process for deepening the case of a case hardened article comprising the steps of coating the article with a material impervious to the constituents comprising the hardened case, heating the article for a period depending on the depth of case desired and subsequently removing said coating.

Another object is to provide a process for the elimination of the white layer in nitrided steel which is simple to perform, easy to control and commercially feasible.

Other objects, advantages and novel details of this invention will be made more apparent as this description proceeds.

In accordance with the present invention applicant provides a method of removing the white nitride layer from nitrided ferrous metal, comprising the steps of coating the metal with an impervious heat resistant material to protect the finish of the metal and to prevent the loss of nitrogen from the metal on application of heat thereto, heating the metal in a controlled atmosphere for a predetermined period and then removing the coating.

The coating may be any material relatively impervious to nitrogen such as copper or bronze having sufiicient heat resistance to Withstand prolonged temperatures of between 850 F. to 1100 F. Bronze and copper electroplating and painting with a tin base paint have produced desirable coatings. The thickness of the impervious coating does not appear to be a factor effecting the predictability of the removal of the white layer from nitrided metal according to the process of this invention. The thickness of the coating must however be suficient to provide an impervious covering on the nitrided metal. A copper plate of about .002 inch thickness has produced excellent results.

The coating performs the function of preventing the denitriding of the nitrided metal on the heating of the metal and also protects the surface of the metal. Previous attempts to remove the white layer from nitrided steel by heating the metal have been carried out without coating the metal as indicated according to this invention. By such heating about one-third of the white layer may be removed from nitrided metal. According to applicants invention heating a coated nitrided metal removes the entire white layer without producing undesirable results. The only known limitations on the coating is that it form a substantially impervious covering over the metal and maintain this covering during prolonged heating. It is of course desirable that the covering be easily removable after heating is completed.

the practice of this invention is between 850 F. to.

1100 F. The time of heating required to completely remove the white layer from coated nitrided steel depends on the thickness of the white layer, the metal constituents, the degree of heat applied and. other factors variable with each case. Roughly the time required to completely remove the white layer from coated nitrided steel according to the present invention is two hours per each ten. thousandths of an inch thickness of the white. layer. The exact time required for complete removal of a white layer by heating a particular coated metal should be de termined by individual tests. The results of a few such tests are given later.

The application of heat to the coated nitrided metal to remove the white layer may be in an air atmosphere or may be in other desiredv atmospheres such as a salt bath. The atmosphere seems to have little effect on the coated metal as would be expected since the coating is relatively impermeable. Particular atmospheres may be desired however to protect the coating at the operating temperatures. Coatings such as copper plate have a tendency to scale due to oxidation at the temperatures most desirable for heating as previously set forth.

A salt bath was used in one instance to prevent such scaling. In the particular case, nitrided metal having a white layer of .0015 inch was given a copper plate of about .002 inch. The plated metal was then heated at 1000- F. for twenty-four hours in. a salt bath. The copper plate was then removed. The white layer was completely removed from the nitrided metal by this process.

Removal of the coating material after the heating of the metal for the required period may be by any known means such as an acid bath or by the use of other solvents for the coating material used. Particular coating removal methods will not be considered here as they will vary with each coating material used and form in themselves no part of this invention.

In the practice of the invention as described above the following are typical examples of the results which have been obtained. In each case the white layer of the indi cated thickness was completely removed from the metal indicated by the method above described.

Nitralloy having the following constituents after nitriding in an ammonia atmosphere in the conventional man: ner was found to have a white layer of .0006 inch. The metal after being coated as described above was heated at 975 F. for fifteen hours. The white layer was completely removed from the metal.

Nitralloy 135 modified:

Element: Percent Mn .40- .70 Si .20- .40 C .38- .45

A1 .85-1.20 Mo .30- .45

Steel of the variety commonly known as 4340 and having the constituents indicated below was found to have a white layer of .0015 inch after being surface hardened by nitriding. Coating the metal and heating it at 975 F. for forty hours effected complete removal of the white layer.

Steel, 4340:

Element: Percent- Mn .60- .80 Si .20- .35

It should be evident from these results that the coating of nitrided metal having a white layer with a relatively impervious heat resistant material before heating to remove the white layer in accordance with this invention is most desirable. Previous heating without such coating caused the elimination of about one-third of the white layer. Heating uncoated fen'ous metal after this point has been reached has been found to produce excessive denitriding of the surface of the metal resulting in a relatively soft surface in which cracks may appear. Heating coated nitrided metal as indicated above eliminates substantially all of the objectionable white layer.

The heating of a nitrided metal coated as indicated above according to this invention also increases the depth of the desirable inner nitride case. in explanation of this desirable result it is considered that the nitrogen freed from the iron nitrides during the elimination of the white layer in the nitrided metal and prevented from leaving the metal by the impervious coating is absorbed further into. the metal where it combines with the alloyingelements in the metal forming nitrides therewith along the crystal surfaces of the iron as in the original nit riding process.

The foregoing specification constitutes a description of the elimination of white layer in nitrided steel in such full, clear, concise and exact terms as to enable any person skilled in the art to practice the invention, the scope of which is indicated by the appended claims.

What I claim as my invention is:

l. A process for eliminating a white nitride layer from nitrided ferrous metal having a white nitride layer, comprising the steps of coating the white layer of the nitrided metal with a heat resistant material which is substantially impervious tonitrogen to prevent loss of nitrogen from the metal on application of heat'thereto, and heating the coatedmetal to a temperature of from 850 F. to 1100 F. for a time sufficient to effect diffusion of the white layer into the ferrous metal.

2. A process for eliminating a white nitride layer from nitrided ferrousmetal having a white nitride layer, comprising the steps ofcoating the white layer of the nitrided metal with a thin metallic coating film which is substantially impervious to nitrogen to prevent loss of nitrogen from the metal on'application-of heat thereto, and heating the coated metal to a temperature of from 850 'F. to 1100 F. for a time sufficient to effect diffusion of the white layer into the ferrous metal.

3; A process for eliminating a white nitride layer from nitrided ferrous metal having a white nitride layer, comprising the steps of coating the white layer of the nitrided metal with a thin metallic coating film which is substantially impervious to nitrogen to prevent loss of nitrogen from the metal on application of heat thereto, and heating the coated metal to a temperature of from 850 F. to 1100 F. for. approximately two hours per each ten thousandths of an inch thickness of the white layer.

4. A process for eliminating a white nitride layer from nitrided ferrous metal having a white nitride layer, comprising the steps of coating the white layer of the nitrided metal with a thin metallic coating film which issubstantially impervious to nitrogen to prevent 'loss' of nitrogen from the metal on application of heat thereto, and heating the coated metal to a temperature of from 850 F. to 1100" F. in a non-oxidizing atmosphere for a time suflicient to effect diffusion of the white layerinto the ferrous metal.

5'. The processof deepening the case of nitrided ferrousrnetal having a white layer, comprising coating the article with a heat resistant material substantially impervious to nitrogen over the surface on which the case is to be deepened, and heating the article at a temperature of approximately 975 F. for a period depending on the depth of case desired, and removing said coating.

6. The process of deepening the case ofnitrided ferrous metal having a white layer comprising coating the 2,960,421 5 6 white layer with a heat resistant material substantially applying heat between 850 F.-'1100 F. to the coated impervious to nitrogen to prevent loss of the nitrogen in meta-1 for approximately two hours for each tel! thouthe white layer on heating of the metal, and decomposing Sandths of an inch thickness 0f the W11ite y the iron nitrides in the White layer of the nitrided metal,

forcing the nitrogen liberated from the decomposed iron 5 References cued m the file of thls patent nitrides of the white layer further into the surface of the UNITED STATES PATENTS metal, and combining the liberated nitrogen with the 2,196,232 Sweeny Apr. 9, 1940 alloying elements in the metal at the greater depth by 2,788,302 Dew Apr. 9, 1957 

1. A PROCESS FOR ELIMINATING A WHITE NITRIDE LAYER FROM NITRIDED FERROUS METAL HAVING A WHITE NITRIDE LAYER, COMPRISING THE STEPS OF COATING THE WHITE LAYER OF THE NITRIDED METAL WITH A HEAT RESISTANT ,ATERIAL WHICH IS SUBSTANTIALLY IMPERVIOUS TO NITROGEN TO PREVENT LOSS OF NITROGEN FROM THE METAL ON APPLICATION OF HEAT THERETO, AND HEATING THE COATED METAL TO A TEMPERATURE OF FROM 850*F TO 1100* F. FOR A TIME SUFFICIENT TO EFFECT DIFFUSION OF THE WHITE LAYER INTO THE FERROUS METAL. 